9th Class Science Force and laws of motion Question Bank Force and Laws of Motion

  • question_answer A bullet of mass \[20\,\,g\] is fired from a rifle of \[8\,\,kg\] with an initial velocity of\[100\,\,m\,\,{{s}^{-1}}\]. Find the final velocity of the recoil of the rifle.

    A) \[-0.25\,\,m\,\,{{s}^{-1}}\]                     

    B) \[2.5\,\,m\,\,{{s}^{-1}}\]

    C) \[25\,\,m\,\,{{s}^{-1}}\]           

    D)        \[250\,\,m\,\,{{s}^{-1}}\]

    Correct Answer: A

    Solution :

     Initial momentum\[=0\] Final momentum\[=(0.02\times 100)+8\times v\] According to the law of conservation of momentum, \[(0.02\times 100)+8v=0\],          \[8v=-2\] \[v=-\frac{1}{4}m{{s}^{-1}}=-0.25\,\,m\,\,{{s}^{-1}}\]

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